1. Field of the Invention
The invention relates to a blanket cylinder of the type having a rotational axis and a concave surface, concentric to the axis and extending over a printing width of the cylinder, for receiving a flat rubber blanket made of elastically deformable material.
2. Description of the Related Art
In rotary offset presses, the printing image is, as is known, transferred from the plate or forme cylinder onto the blanket cylinder, and from the latter onto the paper running over the impression cylinder. It is only possible to transfer the ink, both from the printing forme onto the rubber sleeve and from the rubber sleeve onto the paper, if a certain minimum pressure is present, what is known as the line pressure between the blanket cylinder and plate or forme cylinder.
Here, a problem arises for quality assurance from the demand for ever higher productivity, and as a result of the efforts to produce printing cylinders which are as light and cost-effective as possible. Especially what is referred to as channel-less printing, in particular therefore the sleeve technique, which is distinguished by a printing forme applied without a channel onto a sleeve and/or a rubber blanket applied without a channel, allows the rigidity to be reduced because of the lessened oscillation excitation as a result of the missing cylinder channels. As a result, the length-to-thickness ratio of the printing cylinders, or their relative rigidity with regard to deflection, becomes ever more unfavorable. The consequence of this is that, during printing operation, the shape and position of the printing cylinders with respect to one another change in an undesired manner, i.e. the printing cylinders are deflected.
The positional change as a consequence of a deflection changes the printing pressure, i.e. the setting pressure between the printing cylinders interacting in the printing unit, this setting pressure becoming non-uniform as seen across the cylinder width. This printing pressure is usually determined in numerical values by measuring what is referred to as the imprint width, i.e. the width of the zone which defines the contact area of the cylinders when the cylinders are thrown onto one another, i.e. moved to the pressure position. This measurement is particularly simple in offset printing, since here one cylinder of a pair of cylinders always has a compressible (soft) surface.
As a result of the mechanical misalignment remaining with this, folds can form, as is known, in the conveyed paper web in the press nip of the rubber cylinder if the paper web is moving here with an irregular speed profile across the width because of the above-described positional change, the center of the paper web moving more quickly than the outside of the web, which leads to the formation of folds and hence to printing register problems. In order to counteract printing register problems of this type, various methods are used nowadays. For example, image regulators which influence the web width are used. The image regulators are, as is known, rotating small wheels which press width-reducing furrows into the web. A further method for eliminating this problem is described in DE 44 36 973 A1: the surface geometry of rubber blankets (rubber sleeves here) is configured concavely or convexly across the web width, i.e. a thickness profile varied by the circumferential surface assuming a convex or concave shape on the blanket cylinder in the axial direction of the cylinder. That is to say, concavely ground rubber blankets/rubber sleeves and concavely ground cylinder surfaces, in particular, are described in conjunction with flat rubber blankets/rubber sleeves.
Furthermore, attempts have also already been made to eliminate the above-described printing register problems by means of an appropriately placed support in the form of an inserted film between the blanket cylinder and the rubber blanket. For example, EP 0 704 301 B1 describes a thin intermediate layer for adjusting and improved anchoring of a rubber blanket on a blanket cylinder, in the form of a self-adhesive plastic film. For this purpose, the surface of the blanket cylinder has a region, which is recessed to correspond with the thickness of the thin film, in the form of a diameter reduction.